by George Rhee
Fig. 12.4The JWST orbit. The second Lagrange point (L2), is approximately 1.5 million km from Earth, outside the orbit of the Moon. JWST will orbit L2 with a period of about 6 months (Credit: NASA)
New Technologies
Micro-shutters illustrate how JWST makes use of the latest technologies. These tiny cells about three human hairs wide, have been developed to block out light from bright objects when studying faint nearby object in the sky. The shutters are arranged in a grid the size of a postage stamp that contains about 62,000 shutters. The micro-shutters have lids that open and close when a magnetic field is applied. As shown in Fig. 12.5 each cell can be opened or closed individually to view or blank a portion of the sky. This technology will have applications in fields such as biotechnology.
Fig. 12.5The micro-shutters on the NIRspec instrument operate independently so that JWST can examine many objects at the same time while blocking out light from nearby objects not being targeted. The micro-shutters can be reconfigured for each new observation made in a different part of the sky (Credit: NASA)
The backplane of JWST is a structure that supports the big mirrors of the telescope and also the 2,400 kg of telescope optics and instruments (roughly the mass of two pickup trucks). The structure has to be extremely stable and is made of advanced graphite composite materials and nickel alloy and titanium fillings.
At the cutting edge of infrared technology, the JWST detectors have lower noise and are larger in size than current detectors. The detectors work in a similar way to optical detectors like the ones in your phone. The detector converts the photons (light particles) into electric charge that is collected into individual bins (pixels) within the detector layer. The charge is then converted to voltage signals which are in turn converted into numbers that are stored in computer memory. The device that does this has to operate at the very cold temperatures required of the telescope. The device is called a cryogenic acquisition integrated circuit.
The detectors at mid-infrared wavelengths must be cooled to close to absolute zero. The cooling is important to ensure that the telescope itself does not emit light. Infrared light is heat radiation; if the telescope is not cold it will emit alot of infrared light. The sunshield keeps the telescope cold. It is made of five layers each about as thick as a human hair. The sunshield (Figs. 12.1 and 12.3) works to reduce the 250,000 W that hit the first sun-facing layer to less than 1 W by the time it reaches the fifth layer.
The Instruments
JWST has four scientific instruments: an infrared camera, a near infrared spectrograph, a mid infrared instrument that functions as both a spectrograph and camera and a tunable filter imager.
NIRCam, the near infrared camera takes images of the sky at near infrared wavelengths (from six tenths of a micron to five microns). It will take pictures just as the Hubble Space Telescope did but to far fainter light levels. This camera will detect the earliest stars and galaxies as well as image the star populations in nearby galaxies and search for Pluto-like objects in the outer reaches of the solar system. The camera can block out light to help detect faint objects that are close to bright objects in the sky. The goal of course is to image planets orbiting nearby stars. In fact JWST will be able to detect planets as faint as the Earth at a distance of 25 light years. The camera is being built by a team at the University of Arizona and Lockheed Martin’s advanced technology center. The camera is optimized for the detection of the first stars and galaxies.
NIRSpec, the spectrograph, splits infrared light into its component ‘colors’ or wavelengths. With this instrument, scientists can observe more than 100 objects at once. It makes use of the micro shutter innovation that we described in the previous section. The spectrograph has been assembled in Europe. Many of the astronomical objects of study are so faint that it will take JWST hundreds of hours to capture enough light to form a spectrum. The design of the spectrograph makes it possible to obtain the spectra of many objects simultaneously while blocking out the light of stars and galaxies that would contaminate the spectrum. There is a cloud of dust that surrounds the Earth and Mars that emits infrared light. The shutters will also serve to block out this so-called zodiacal light so the spectrograph can reach fainter objects.
MIRI, the mid-infrared instrument covers the range from 5 to 28 μm. A micron is one millionth of a meter (about the size an E. coli bacterium). The instrument is designed to take images and spectra which can be used to distinguish between very distant galaxies that are forming stars for the first time and those that have ongoing star formation. The surveys for young galaxies will be made with the infrared imager to find candidates through the dropout technique. The imager will then sift through these high redshift galaxy candidates and find galaxies forming the first generation of stars. This will extend the known high redshift galaxies out to redshifts of about 15, where we are seeing objects as they appeared a mere 300 million years after the big bang, where no object is currently known.
The fourth instrument is the tunable filter imager. It will take images at very specific wavelengths in the range 1–5 μm. The tunable filter imager is packaged with the fine guidance sensor which is used to provide guide stars and point the telescope accurately. The guide stars are used to keep the telescope pointing at exactly the right part of the sky for a given observation.
Rocket Science
Figure 12.6 shows the Ariane Five rocket that will launch JWST. The rocket will carry the telescope 1 million miles from Earth, more than four times the Earth-Moon distance. The Hubble Space Telescope by contrast orbits a mere 350 miles above the surface of the Earth.
Fig. 12.6The Ariane 5 rocket pictured above will launch JWST into space. The main telescope mirror will measure 6.5 m in diameter, too large to launch in one piece. It will consist of 18 individual mirror segments mounted on a frame which will be folded inside the the Ariane 5 at launch. The right panel shows the a closeup of JWST stowed inside the Ariane 5 (Credit: Arianespace – ESA – NASA)
The rocket has a length of about 50 m and weighs about 800 tons at liftoff. These rockets are often used to carry telecommunication satellites to geostationary orbits. They are also used to send resupply spacecrafts to the International Space Station.
The rocket propulsion system consists of a main cryogenic stage and two solid booster stages. The main cryogenic stage carries the propellant, liquid oxygen and hydrogen, for the main engine. The engine runs for about 10 min until the fuel is used up. The cryogenic stage then reenters the atmosphere and splashes down in the ocean. The rocket also has two boosters which provide 90 % of the total thrust at the start of the flight and for about 2 min before they separate over a designated zone in the Atlantic ocean. The rocket has had more than 200 launches in the past 30 years and 44 launches in a row over the past 8 years. These safety criteria are quite different to those used for automobiles. Would it be a strong selling point if you were told a car had made 40 successful trips without exploding?
As we have mentioned, JWST is stored folded inside the rocket. Four days after launch, the deployment of the antennas, solar array and sunshield will be completed. The spacecraft will arrive at the L2 point after about 1 month. After deployment the equipment is tested during the journey to L2. About 28 days after launch the telescope starts to be cooled down. Once the telescope is sufficiently cold preliminary science observations take place. There is a commissioning period of several months to make sure that system performance is closely understood. The goal is for JWST to operate for at least 5 years after completion of commissioning but JWST has the potential to keep operating for 10 years since it carries enough propellant to keep in orbit for that time. The propellant is required because L2 is what is known as a saddle point. The analog on Earth is a saddle on a mountain such as the famous South Col of Mount Everest. On a smooth version of the South Col a bowling ball will start to roll towards the Lhotse face or the Kanshung face and fall off the mountain, the same is true of JWST at L2. For this reason JWST is not located act
ually at L2 but orbits around that location. The light from the Sun exerts pressure on the sunshield causing the telescope to spin and thruster firings are required to counter this.
JWST will be operated from the Space Telescope Science Institute on the campus of Johns Hopkins University. 10% of the observing time on the telescope has been awarded to the science instrument teams and 5% will be the directors discretionary time. It might seem strange to award the director a substantial amount of time to do whatever he or she wants without peer review. Let us recall that the Hubble Deep Field project, arguably the Hubble Space Telescope’s most successful and well known observation, was carried out using discretionary time. The remaining 85% of the observing time will be awarded through peer reviewed proposals, any astronomer in the world can apply.
JWST Versus Ground Based Telescopes
In the past the Hubble Space Telescope has been used in conjunction with ground based facilities such as the Very Large Array in New Mexico and the Keck Observatory in Hawaii. The Keck telescope was used because its much larger mirror makes it more effective than Hubble for faint object spectroscopy. Hubble data were also combined with the Spitzer Infrared Observatory to constrain the mix of stars that are present in distant galaxies. There are several cutting edge facilities that will be working at the same time as JWST. The Atacama Large Millimeter Array (ALMA) observes at longer wavelengths than JWST. The 30 m class optical telescopes such as the ESO ELT, the Giant Magellan Telescope and the Thirty Meter Telescope will be operating at the same time as JWST. These three large telescope projects are moving forward from design to construction phase. If adaptive optics works as advertised, these telescopes will deliver images that show more details than JWST images of the same objects. It is difficult to predict exactly how the 30 m ground based telescopes will complement the JWST since the exact specifications of each are still uncertain, but the telescopes will be used to study the origin of galaxies in different ways. There is also the problem of funding; there is not enough money available as of this writing to fund all three 30 m class telescopes. The ground based telescopes have the advantage of a large collecting area and high angular resolution. The advantages of JWST are high quality imaging over a large field of view, not limited by atmospheric absorption. JWST will function continuously and not be subject to night and day and weather constraints. JWST will also have an advantage at wavelengths larger than 5 μm where the background emission is high for ground based telescopes.
Politics: The Art of the Possible
JWST is expensive, the cost to launch is 8 billion dollars. A debate took place over whether to cancel the whole project. On June 29, 2010 Senator Mikulski of Maryland wrote a somewhat strident letter to NASA stating
I am deeply troubled by the escalating costs for JWST. The report the agency provided in response provided little comfort that the problems are behind us. I request that you immediately initiate an independent and comprehensive review of JWST led by experts outside of NASA.
On July 27, 2011 when the house appropriations committee proposed a bill to terminate JWST Senator Mikulski released the following statement
Today the House Appropriations Subcommittee on Commerce, Justice, Science and Related Agencies passed a bill that would terminate the James Webb Space Telescope, kill 2,000 jobs nationwide and stall scientific progress and discovery. It was a shortsighted and misguided move.
The Webb Telescope will lead to the kind of innovation and discovery that have made America great. It will inspire America’s next generation of scientists and innovators that will have the new ideas that lead to the new jobs in our new economy.
The Administration must step in and fight for the James Webb Telescope.
The Senator took little comfort in the NASA response to her concerns, yet she felt even more uncomfortable at the prospect of the project being canceled. This returns us to the question we asked at the beginning of the last chapter; How much should we as a society spend on basic research? How do we know we are getting value for money? There is a fantastic youtube video by Hank Green of the Vlogbrothers entitled “The top five awesome things about the Webb telescope”. It is on talented science advocates such as Hank Green that the future of astronomy depends. They have the ability to convey to a large audience the value and excitement of science which in turn ensures that the adventure can continue.
Further Reading
The James Webb Space Telescope. J. Gardner et al. Space Science Reviews. 2006, volume 123, pp 485–606.
Cosmic Discovery. M. Harwit, Massachusetts Institute of Technology Press, 1984.
Top Five Awesome Things About the Webb Telescope. H. Green (http://www.youtube.com/watch?v=ihpNNBmJypE)
George RheeAstronomers' UniverseCosmic Dawn2013The Search for the First Stars and Galaxies10.1007/978-1-4614-7813-3© Springer Science+Business Media, LLC 2013
Epilogue
This book is in some sense a play in three acts. In the first act entitled Prologue we presented the dramatis personae, the main characters in our play. These were the galaxies, stars and dark matter which comprise our universe. We also set the scene by reviewing the development of cosmological ideas up to the early twentieth century. We presented the Big Bang theory and the observations that support it.
In the second act of our play entitled ‘The Emergence of Galaxies’ we set up the plot or dramaturgy of our story. We reviewed our theories of galaxy formation and the related observations. In Chap. 5 we presented the evidence that galaxies form a cosmic web that spans hundreds of millions of light years. With the discovery of these structures it is natural to ask how they formed and this was the topic of Chap. 6. We examined the emergence of galaxies from small regions of slightly higher than average density in the early universe. We also discussed how the cosmic web itself emerged from these small density variations. In Chap. 7 we showed how the size of these small variations in density is estimated using observations of the cosmic background radiation. We also use the cosmic background radiation measurements to estimate the density of the universe and its geometry. Then the plot thickens. Together with supernova measurements we find to our surprise (physics Nobel prize 2011) that the universe is expanding faster and faster. This is evidence for a new force of nature known as dark energy.
In our third and final act we took the reader to the frontier of cosmology where astronomers are using computers and telescopes to solve the riddle of galaxy formation. We began with the planned observations of the dark ages, a time before stars and galaxies existed, just the silent condensation of hydrogen gas in clumps of dark matter. We hope to detect the changes in the hydrogen gas caused by the light emitted as the first stars came into being. We then explored the distant galaxy frontier in Chap. 9. This is the attempt to push back towards the dark ages by observing galaxies at increasing distances from earth. With our current technology we can see light emitted by galaxies 13.2 billion years ago, we hope to push this boundary back another 500 million years closer to the Big Bang itself.
In Chap. 10 we discussed the possibility that fossils of the first galaxies may be lurking right on our doorstep so to speak. These may be very small galaxies containing only a few hundred stars that formed over 13.5 billion years ago, the oldest known objects in the universe. The evidence so far is statistical but direct confirmation of a galaxy fossil may come soon. These three approaches should detect the cosmic dawn and yield a picture of the formation of the first stars and galaxies.
The telescopes that will take us on this journey to our cosmic origins are truly revolutionary. They will all be operational within the next decade. The Square Kilometer Array radio telescope will image the neutral hydrogen clouds in the dark ages. The Large Synoptic Survey Telescope will produce a very faint image of the whole southern sky. The 30 m class optical telescopes will push the distant galaxy records and make it possible to explore the distant galaxy universe. We hope to understand the nature of the first objects to light
up the universe, beyond just establishing their existence. The James Webb Space Telescope, successor of the Hubble Space Telescope, is expected to play a major role in this endeavor.
We also hope and expect to see the unexpected; things ‘undreamt of in our philosophy’. I hope this book has conveyed the excitement of this adventure. I hope it is a good starting point for a reader to go out and learn more of the endeavors of the one in 100,000 humans on our planet who earn their living looking up at the sky.
Glossary
AnisotropyA deviation from perfect uniformity
BaryonGeneric name for a neutron a proton or a quark
Big BangThe explosive event in the very early history of the universe that led to its current expansion and structure
BlackbodyA system of radiation and matter in which the latter emits as many photons as it absorbs
